Astronomers Discover Stars With Carbon Atmospheres. Released at November 21, 2007 P. Dufour, J. Liebert, G. Fontaine, N. Behara published the results in the Nov. 22 issue of Nature. Introduction. Normal white dwarf : the DAs and the non-DAs. George P. McCook and Edward M. Sion,

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Astronomers Discover Stars With Carbon Atmospheres

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Hot DQ stars

Most stars are very similar to the previously known DQ stars G227-5 and G35-26 and exhibited mostly CI lines.

a few showed a more complex and rich spectrum that is attributed to CII absorption lines.

no combination of carbon and helium could successfully reproduce the observed features by assuming a helium-dominated atmophere

Fit to the optical spectra (left panel) and energy distribution (right panel) for carbon-rich white dwarfs with log g fixed at 8. Hydrogen and helium abundances are constrained (or dermined in two cases) from the absence of the H(4861) and HeI 4471 lines.

Photometric distances, based on the assumption that log g = 8, are indicated

in each panel.

H1504+65 and evolutionay scenarios

these WDs are the progenies of objects such as the unique hot PG1159 star H1504+65

Artists' concept of the surface of the white dwarf star H1504+65, believed to have somehow expelled all its hydrogen and all but a very small trace of its helium, leaving an essentially bare stellar nucleus with a surface of 50 percent oxygen and 50 percent carbon. When this star cools, it may have a carbon atmosphere.

H1504+65 and evolutionay scenarios

It could also be the result of a heavy-weight intermediate-mass star (8 M⊙≤M≤10M⊙) that has gone through carbon burning and has a O-Ne-Mg core.

for some stars, data fits with models with high surface gravity have lines that are much too broad

Conclusion

All carbon-rich white dwarfs they have found so far have Teff between 18,000 and 24,000 K

a star like H1504+65, showing initially a mixed C and O atmosphere, would temporarily disguise itself as a helium-rich star before transforming itself into a carbon dominated atmosphere star at an effective temperature whose exact value is not yet known due to a current lack of proper models (hopefully around 24,000 K !)

these stars must again go through a drastic spectral change as they cool